HVAC Heat Pump
 

Technology Connections just released this video explaining that furnaces are massively oversized, and several methods to see how oversized your furnace is.

https://www.youtube.com/watch?v=DTsQjiPlksA

My furnace is a single-stage, meaning it's either on, or off. This makes calculating how oversized it is easy. On the coldest day this year (Jan 13), we hit a low of 16F and a high of 24F (about the coldest it gets here). I reviewed my furnace fan power consumption for that day, which came in at 5.56 kWh. My fan runs at almost exactly 700 watts, and only runs when the furnace is on.

My weather station
https://ambientweather.net/dashboard...b5b0a74/graphs

If my furnace ran at 100% capacity (on for 24 hours in the day), the fan would consume 16.8 kWh (24hrs x 700 watts).

So, my duty cycle on the coldest day is 5.56 kWh / 16.8 kWh = 30%

The furnace is rated at 100,000 BTU/hr, so 30% of that is 30,000 BTU/hr. That means that a 2.5 ton (12,000 BTU = 1 ton) heat pump would have just maintained heat running at 100% capacity.

The real problem I'm trying to solve is to even the temperature differential between upstairs and downstairs. It's typically 6-10 degrees cooler downstairs. With my furnace blower only running 10% of the time on average during the coldest month, there's not much mixing occurring.

My thought is to replace my 5-ton, 10 SEER AC unit and 80% efficient gas furnace/blower with an inverter heat pump. Inverter heat pumps can run at any output from about 35% to 100%. They are designed to run continuously instead of turning on and off. My assumption is that running continuously will keep the air mixing between upstairs and down, somewhat evening out the temperature differential. My other assumption is that I can close registers to force more heat downstairs without risk of stressing the ductwork since this system would be running substantially less pressure.

Other minor benefits include substantially less noise from the blower and furnace.

Other thoughts are to install a door between upstairs and down, and to increase soffit ventilation since mine is such a joke, leading the attic to get very hot in the summer.

It's like deja vu all over again.

That's the first thing I remember you posting on the forum.

Well, I joined in 2010, and bought this house in 2019, so there was 9 years of yammering about other things.

First thing I remember posting was a physics question regarding hill climbing. All engine efficiency and aero factors aside, is it more efficient to go faster up a hill. It takes a minimum amount of fuel burn to merely maintain position on a hill, and doing so would be 0% efficient. So you've got to burn the minimum plus something to go up it. The faster the hill is overcome, the less minimum fuel burn is needed. The analogy I gave was a rocket hovering, vs barely gaining altitude, vs going as fast as it can.

I spent too much time estimating how many BTUs I use for heating, converting that to therms, calculating my heating gas bill based on that, figuring out how many kWh I'd use if I heated with a 10 HSPF inverting heat pump... my estimate is I would save between $0-$100 per year. Very tricky converting between all these units. The SEER and HSPF should be simple % efficiency ratings (like a gas furnace) so the math is easy.

Anyhow, I estimated my furnace is on 6% of the year based on my blower on time, which amounts to 657 total heating therms. I also looked at my monthly gas bills, and knowing I have zero heating in July, Aug, Sept, I was able to roughly determine I use 20 therms/mo in hot water, range, dryer. Subtracting that from the total for last year, it comes to 665 therms used for heating.

I pay about $865/yr to heat on 80% efficiency gas. I estimate $790 heating on an inverting heat pump.

By first I meant earliest.

You hill-climbing example, is like mine of getting from a stoplight at 0MPG to an efficient cruise.

For seven years, I lived in a geodesic dome with a ceiling fan and a hot-water radiant floor under ceramic tiles. I found that optimal.

I haven't watched the video, but in my situation I let the house cool off at night and also when I'm not there. It can take several hours to get the temp back up. So an oversized furnace is a good idea in my mind.

I'm not sure if I'd save any money if I had a heat pump since gas is about a third the price of electricity.

Here's 1 thread where I talked about this;

https://ecomodder.com/forum/showthre...ing-40669.html

and another;

https://ecomodder.com/forum/showthre...irs-37573.html

and another;

https://ecomodder.com/forum/showthre...ing-40669.html

You can tell I'm finally getting serious about this problem.

I'm not a pro, but I have spent some time on HVAC-TALK and I am facing some of the same issues you discuss.
Regarding closing off returns... you SHOULD familiarize yourself with TESP measurements (static pressure in ducts) This is easy, but you need to buy, borrow or make a manometer. You also need to verify the TR (temperature rise from return (intake) air and supply duct air temperature. Compare that to the label on the furnace.
Fan speeds... you may be able to lower the fan speed for heat... ( but, not if you don't understand the function/purpose of the above two easy to preform tests.)
Do you know what your fan speed is? For example, a 80,000 btu furnace should have a minimum of 20x25 inch duct.....
What is the size of your return duct... should be equivalent to 16" round for a 80k btu and 1200 cfm.

Are the ducts rigid or flex?

Furnaces usually have a separate speed for heat and a faster speed for A/C....

You may save money hiring a HVAC guy to look over your ducts... the purpose of such is to "prep" your current ducts to a new system you 'might' buy. Since your current furnace is oversized, it's possible ducts will be efficient when you decide to buy a 50k btu unit.

And, it's possible....you might spend lots of money/time and not solve your temperature stratification cheaply...( newer units will have more efficient blowers that operate more cheaply (guess around 30%?) if you want/need to run the fan more often.

Good advice. Seems simple enough.

Don't know my fan speed. My furnace is 125,000 BTU, with 100,000 of useful heating. I have 2 returns shown in this photo. The cylindrical vertical one is from the upstairs ceiling, and the other is on the floor of the basement with the filters sitting on top.

https://www.diychatroom.com/cdn-cgi/...07-jpg.781672/

Flex branches; each one going to an individual register.

If mine does, it's not reflected in power consumption. I would think less speed is needed for AC considering 5 tons of cooling is nothing compared to 100,000 BTUs of heating.

One person suggested installing more registers both upstairs and down, and that way some of the upstairs/downstairs ones can be closed depending on the season. I basically need all of my cooling to come from upstairs in the summer, and 90% of heating to come from downstairs in the winter. I've got 10 registers upstairs, and 7 down.

Here's the downstairs, with 2 bedrooms and a bathroom not pictured (behind). The white door on the left leads to the utility room where the furnace is.
https://www.diychatroom.com/cdn-cgi/...53-jpg.781670/

The rest of the single main supply plenum
https://www.diychatroom.com/cdn-cgi/...10-jpg.781671/

Could you post a picture of the furnace spec plate?
ECM motors (variable speed) can be installed... (google time)....part costs less than $300.

Assumption...not enough return air. Could you provide:
1. Measurements of both return ducts (ie: 16" or 20x25")
2. Measurements of the return air grilles
3. Roof insulation depth or R-value. ( mine was R19; 8 inches fiberglass, I went balls out and it is now R70,;24-30" )
4. Type of attic vent, est size of vent, soffit ( opinion as to whether attic ventilation can be improved?)
5. Opinion on noise level from air rushing into return grilles. ( too noisy? )

The furnace must be sized for unusualy cold weather events unless you have robust backups.